1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an optical modulator including a thin plate having an electrooptic effect and having a thickness of 20 μm or less and a reinforcement plate having a thickness lager than that of the thin plate.
2. Related Art Statement
In the related art, waveguide-type optical modulators having optical waveguides or modulation electrodes formed on a substrate having an electrooptic effect have been widely used for optical communication fields or optical measurement fields.
Particularly, an amount of information transmission increases as multimedia technology is developed. Therefore, there is a need for a wide frequency band modulation technology in an optical modulator. In order to implement the wide frequency band modulation, various external modulation schemes with an LN (lithium niobate) modulator or the like are used. However, in order to implement the wide frequency band modulation with the LN modulator, there is a need for velocity matching between an optical wave and a microwave, that is, a modulation signal and decrease in driving voltage.
As one of means for solving the problem, a technique of satisfying the condition of velocity matching between the optical wave and the microwave and decrease in the driving voltage by using a thin substrate has been conventionally known.
In JPA S64-18121 and JPA 2003-215519, an optical waveguide and an modulation electrode are disposed in a thin substrate (hereinafter, referred to as a first substrate) having a thickness of 30 μm or less, and after that, a substrate (hereinafter, referred to as a second substrate) having a dielectric constant lower than that of the first substrate is attached to the first substrate. By doing so, an effective refractive index with respect to a microwave is reduced, so that velocity matching between the microwave and an optical wave is obtained, and a mechanical strength of the substrate is maintained.
In JPA S64-18121 and JPA 2003-215519, the first substrate is mainly made of LiNbO3 (hereinafter, referred to as LN), and the second substrate is made of a material having a dielectric constant lower than that of the LN, such as quartz, a glass, and an aluminum oxide. In the combination of these materials, due to a difference between linear expansion coefficients thereof, temperature drift or DC drift occurs according to change in temperature. Particularly, in JPA 2003-215519, in order to remove such defect, the first and second substrates are attached to each other by using an adhesive having a linear expansion coefficient close to that of the first substrate.
However, in comparison to an optical modulator fabricated by using the conventional LN substrate, an optical modulator fabricated by using a thin-plate LN substrate has a larger amount of coupling loss between an optical fiber and an optical waveguide formed on the substrate. As a result, insertion loss of the optical modulator increases.
Therefore, the inventors have done researched to determine the cause of the increase in the coupling loss between the optical fiber and the optical waveguide formed on the substrate. Finally, the increase in the coupling loss is clarified to be caused from a change in light propagation mode distribution due to a thickness of the substrate. More specifically, if the thickness of the substrate is reduced, a horizontal light confinement of the optical wave propagating along the optical waveguide in the horizontal direction (that is, a direction perpendicular to the thickness direction of the thin plate) is weakened, so that the light distribution spreads in the horizontal direction. Particularly, if the thickness of the substrate is reduced down to 20 μm or less, a vertical light distribution, that is, a light distribution in a vertical direction (that is, the thickness direction of the thin plate) has a large difference from the horizontal light distribution. Moreover, if the thickness of the substrate is reduced down to 10 μm or less, the difference between the vertical and horizontal light distributions increases greatly.